Hello kinza82,
Do you have change with buttons enabled? If not, we'd recommend testing that out now: Adjust the volume on iPhone - Apple Support
Adjust the volume on iPhone
When you’re on the phone or listening to songs, movies, or other media on iPhone, you can use the buttons on the side of your device to adjust the audio volume. Otherwise, the buttons control the volume for the ringer, alerts, and other sound effects. You can also use Siri to turn the volume up or down.
Siri: Say something like: “Turn up the volume” or “Turn down the volume.” Learn how to use Siri.
WARNING: For information about avoiding hearing loss, see Important safety information for iPhone.

Lock the ringer and alert volumes in Settings
1. Go to Settings
.
2. Tap Sounds & Haptics (on supported models) or Sounds (on other iPhone models).
3. Turn on Change with Buttons.
Adjust the volume in Control Center
When iPhone is locked or when you’re using an app, you can adjust the volume in Control Center.
Open Control Center, then drag
.
This should result in the ability to change the volume with the iPhone's side buttons. Are you now able to adjust the volume using the volume buttons on the side of your iPhone?
If not, we'll need to ask some questions to get a better understanding of what's going on. Do you have this issue in other apps? Does it occur when using headphones or Bluetooth speakers? Does the other person on the FaceTime call hear the same on their end?
Hoping to hear back, take care.